The present invention relates to the construction of a rotary drum assembly for video tape recorder (VTA).
The rotary drum assembly for a VTR must be constructed so that each of the magnetic heads supported thereon can be actually brought into contact with a magnetic tape accurately at a set position on the magnetic tape and at a fixed contact pressure. Accordingly, the eccentricity of the rotary drum of the rotary drum assembly supporting the magnetic heads must be reduced to the smallest possible value. The eccentricity of the rotary drum in the present state of the art is on the order of several micrometers. In a conventional rotary drum, as shown in FIG. 7, to limit the eccentricity of the rotary drum to such a small value, magnetic heads 10 are supported on a rotary drum 9 fitted on a disk 1 mounted on a spindle 5 by press fitting. The spindle 5 is supported on bearings 7 on a stationary drum 2 disposed beneath the rotary drum 9 and is rotated at a high rotating speed by a driving motor, not shown. The eccentricity of the rotary drum 9 ca be reduced by improving machining accuracy for the mating surfaces A of the disk 1 fitting to the mating surface of the rotary drum 9, and improving machining accuracy for the rotary drum seating surfaces B of the disk 1 determining the axial position of the rotary drum 9, namely, the height of the rotary drum 9 from a datum plane. For example, the disk 1 is finished so that the eccentricity of the mating surfaces A is in the range of zero to several micrometers and the rotary drum seating surfaces B is finished with a desired high accuracy in respect of runout and the axial position of the rotary drum. The mating surface of the rotary drum 9 mating with the mating surfaces A of the disk 1 is finished in a transition fit or an interference fit, and the rotary drum 9 is attached to the disk 1 by pressing using a jig. Thus, the eccentricity of the contact surface 7 of the rotary drum 9 to be in contact with a magnetic tape is limited to a value below several micrometers. No problem arises in the conventional rotary drum assembly thus constructed when incorporated into an ordinary VTR, such as a domestic VTR which does not require the replacement of the rotary drum 9 with a new one, which is necessary when the magnetic heads 10 are abraded and the service life of the rotary drum 9 is terminated, because the VTR is not that frequently used. However, a VTR for business purposes which is used very frequently, such as a VTR for broadcasting business, requires frequent change of the rotary drum 9 upon the termination of the service life of the magnetic heads 10 due to abrasion, and ease in replacing the rotary drum 9 with a new drum is one of the important requirements of the business VTR. Accordingly, the mating surfaces A of the rotary drum of a business VTR are finished with an accuracy corresponding to a low-class running fit to give the highest priority to preventing the seizure of the rotary drum 9 by the disk 1 and to facilitate removing and replacement of the worn rotary drum 9. Drum on the disk 1. Consequently, the eccentricity of the contact surface of the rotary drum 9, as mounted on the disk, is increased and hence skill and much time are required for correcting the eccentricity of the rotary disk.